Moderators of respiratory muscle function in health, exercise and disease

Abstract

This research presents a series of studies investigating the role of the respiratory muscles in both healthy and diseased populations. 

To test directly the role of the respiratory muscles in a healthy population various strategies were used. Two studies examined the effects of training these muscles to bring about improvements in inspiratory strength and endurance and observe the effects of these changes on whole body performance. 

Following a 10-week intervention of inspiratory resistive loading (IRL) study one demonstrated a 34% increase in maximum inspiratory pressure and a 36% increase in cycling endurance capacity at 75% V̇O₂peak (increased time by 1292 ± 607 seconds). During the trial heart rate (HR), ventilation (V̇E), and ratings of perceived exertion (RPE) were significantly reduced at the same time points.

Study two examined the long-term effects (3 months) of non-invasive positive pressure ventilation (NIPPV) use in type II respiratory failure patients (PaO₂ <7.3 kPa & PaCO₂ >6.5 kPa). This treatment modality improved daytime blood gas tensions (PaO₂ increased to 8.06 ± 1.24kPa; PaCO₂ decreased to 6.45 ± 0.73 kPa), oxygen saturation (increased to 89.9 ± 3.7%), bicarbonate retention (decreased to 25.8 ± 3.86) and patient perception of health (as measured by St George’s Respiratory Questionnaire) in a group of patients receiving otherwise optimal treatment. In addition, both inspiratory muscle strength and endurance were improved relative to baseline following the three month intervention.

Prevention of loss in body mass, particularly in the lean body compartment, is a potential method of alleviating muscle dysfunction in Chronic Obstructive Pulmonary Disease (COPD). Therapeutic doses of anabolic androgenic steroid (AAS) administration have been shown to increase maximal inspiratory pressure. Study three was designed to investigate the long-term effects of taking AAS (>20yrs) in a population of bodybuilders to help determine the safety aspect of the drugs with specific application to the respiratory system. Spirometry was within normal range and there were no differences between a group using AAS, a group abstinent from AAS use for three months; bodybuilders who had never used AAS; and sedentary controls. Maximum inspiratory pressure (MIP), a gauge of inspiratory muscle strength, and grip strength were both significantly greater in the group using AAS. MIP was 148 ± 24 cm H₂O in the AAS using group compared to 117 ± 26 cm H₂O in the bodybuilding control group. Although the use of AAS is associated with a variety of potentially hazardous consequences on cardiovascular and hepatic health, we demonstrated no adverse effect on respiratory (muscle) function, lung volumes or indices of flow, or breathing profiles.

Investigating the use of AAS provided an opportunity to examine a subset of bodybuilders self administering recombinant human growth hormone (rhGH). As a potential means of improving body composition and respiratory function in cachectic COPD patients, study four examined the effects of short-term high dose rhGH administration on body composition and respiratory function. Male subjects self administered 0.019 mg.kg^-1 .day^-1 for six days. After this intervention maximum inspiratory and expiratory pressures (MIP & MEP respectively) significantly increased when compared with a control group, (MIP144 ± 24 cm H₂O ; MEP 179 ± 35 cm H₂O). Increased body mass index and fat free mass were observed with a decrease in body fat when compared with a control group (Body fat pre = 20.0 ± 6.0%, during = 19.0 ± 6.0%, post = 19.1 ± 5.8%.

Differing results from various published studies may be related to the intensity of the intervention. Study five compared two training protocols, the first set at 80% of maximum (SUB), and a second at 100% of maximum (MAX), a third control group performed no training (CON). Both training protocols brought about improvements in inspiratory muscle performance (increases in MIP of +32 ± 19 cm H₂O for MAX and +37 ± 25 cm H₂O for SUB) but only the maximally trained group demonstrated a decrease in exercising heartrate (-6 ± 9 beats.min^-1) and RPE (-0.5 ± 1.4).

Study six, examined stroke volume during respiratory manoeuvres against resistance, and was undertaken as a pilot investigation to ascertain possible mechanisms via which improvements in whole body exercise can be produced through respiratory muscle loading. Stroke volume during supine rest (~ 100ml) was greatly decreased during a maximal inspiration against resistance (~80ml) and very quickly increased (~110ml) on cessation of this manoeuvre. Chronic exposure to these manoeuvres, which are a feature of an IRL intervention, potentially has a profound effect on the heart through the Frank-Starling relationship. These interesting results require further investigation.